UK light-heavyweight Buatsi made a winning yet underwhelming beginning to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a fight where the approaches failed to mesh, with frequent clinching disrupting the fight.
The officials had it 96-94 for to the winner, with the third a even score.
"It wasn't my best showing but what defines champions? They prevail," the victor said in the arena as he was met by disapproval from the audience.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts felt Parker, thirty-one, merited the win but the contender experienced the second loss of his 28-match professional journey.
Buatsi Secures Triumph in Slow Encounter
His career development has plateaued in recent times through contract problems and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a last chance - under new promotional backing.
He opened tentatively, with neither fighter willing to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second fight at light heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for Buatsi, who utilized a solid defense to narrowly win the cleaner rounds.
The spectators' acceptance wore thin amid constant clinching, with the challenger going down several times during grappling.
The underdog closed well, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Hatton Memorial Inspires All-Manchester Contest
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a terrific local rivalry contest to capture the European light heavyweight championship.
The arena fell silent for a deeply poignant homage prior to the bout as 10 bells rang in remembrance of local hero Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the last bell ended, the silence gave way to a roar of "there's only one Hatton".
The action during the bout reflected the feeling. Arthur was floored by a powerful punch flush on the chin in the initial stage, getting up unsteadily to his feet on the eight count.
But the red-colored shorts of Arthur soon pressed ahead again. Across the ring, Rea - in competitive blue - was injured quickly, heavy swelling appearing under his orbital area as the fighters traded punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the intensity rarely dropped over twelve rounds.
Arthur narrowly won the decision with judges scorecards of 114-114, close margin and 115-112, but both fighters were persistent and courageous, exactly what their inspiration the legend stood for.
